Oscillators, which are generally voltage controlled but may be current controlled, are commonly used in modern electronic circuits, for example in frequency synthesizers, in order to deliver regular signals over time to a computer or to a data stream. Inductor-capacitor (LC) oscillators are of sinusoidal type and usually include an oscillating circuit comprising a coil and a capacitor. The oscillating circuit sets the oscillation frequency and stores the energy of the oscillations.
Phase noise is an important parameter of frequency synthesizers and oscillators and represents the random fluctuations in the phase of a waveform.
In ultra-high-speed data transfer technologies it is particularly advantageous for the phase noise of the oscillators to be extremely low, especially for millimeter wavelengths. Millimeter wavelengths are, for example, used in telecommunications systems such as ultra-high-speed data backhaul networks, in automotive radar or else in medical or military radar.
Currently, integrated on-silicon technologies do not meet the needs of the latest generation of backhaul networks, and low phase noise LC oscillators are usually produced using III-V semiconductor technologies such as those based on gallium arsenide GaAs.
However, III-V-semiconductor-based technologies are more expensive and provide less in the way of functionality compared to silicon-based technologies.
In general, phase noise usually decreases logarithmically with the increase in the energy held within the resonant circuit; more specifically, increasing the energy by a factor N decreases the phase noise by 10×log 10(N).
However, to increase power to decrease phase noise requires larger transistors, which imply more parasitic capacitance and therefore smaller frequency ranges, and therefore a lower inductance, and therefore a lower quality factor and therefore an increase in phase noise.
This iteration reaches a limit when it is physically impossible to decrease the inductance any further, and the phase noise is improved at the expense of the extent of the range of frequencies, which is no longer enough to cover the range set by the telecommunications standard.
An experiment carried out on phase noise in MESFET (GaAs) oscillators coupled, in a non-integrated manner, by transmission lines (Chang, et al., “Phase Noise in Coupled Oscillators: Theory and Experiment”, IEEE Transactions on Microwave Theory and Techniques, 1997, pp. 604-615, incorporated by reference) was carried out and demonstrated that coupling between multiple oscillators may allow their powers to be combined and hence phase noise to be decreased on condition that these oscillators are coupled in a reciprocal and bilateral manner.
Problems with this “macroscopic” experiment, i.e. one not carried out on an integrated circuit, include, in particular, frequency correspondence between coupled oscillators, phase shift tuning, or phase noise that is sensitive to synchronization.
On the other hand, United States Patent Application Publication No. 2013/0099870 (incorporated by reference) describes a system of coupled resonant circuits allowing a low phase noise to be obtained. However, the coupling connections are formed through sharing the tracks of the inductive elements (coils) of two complementary resonant circuits in contact.
In this configuration, certain circuits, for example the circuits located at the edge of the system, do not have the same resonant frequency as the others. Moreover, the components and power supply lines are placed within the coils, which is known to be detrimental to the performance of the system.
Thus, it would be advantageous to make use of the logarithmic improvement in phase noise obtained by increasing the energy held, without having to modify the structure of a functional oscillator circuit.